92 CLASS MAMMALIA. 



the bats the most astonishing complications, and even the 

 heart is in some measure displaced, and situated higher up 

 in the cheiroptera. The pectoral muscles strongly experience 

 this influence ; they are more voluminous, and have their 

 seat and points of attachment on a sternum, composed of 

 pieces remarkable for their size and perfect ossification. 

 The sternum of the quadrumana, on the other hand, is 

 weak, small, and almost entirely cartilaginous. 



In the last mentioned animals the bones of the fore-arm 

 are susceptible of pronation and supination, an immense 

 advantage to animals designed to live in trees ; but this 

 faculty would prove a serious inconvenience to the bats, as 

 at every flapping of their wings, the resistance of the air 

 might occasion a rotation of their hands. They are, there- 

 fore, deprived of it. This is accomplished by the sacrifice 

 of the cubitus, which, however, does not entirely disappear. 

 The tertius humeralis remains, and this portion connected 

 to the radius, contributes to give it sufficient force and 

 solidity to sustain the carpus and the entire hand. 



When we compare the anterior and hinder extremities of 

 the bats, we may calculate in some measure the wonderful 

 augmentation of the former. The latter remain within or- 

 dinary dimensions, and are but partially involved in the 

 membranes of the flanks. The foot is free. The interfe- 

 moral membrane has its final points of attachment on the 

 tarsus, and is sustained when in a state of developement by 

 one of the small bones of that part, projecting outwards in 

 a spiral form. 



The hinder toes are small, compressed and equal, and 

 always five in number. The thumb is indistinguishable. 

 All are terminated by claws or little horny laminae, formed 

 like the quarter of a circle, very sharp at the point, and re- 

 markable from their equality and parallelism. 



This conformation of the toes enters of necessity into the 

 constituent plan of these animals, and never experiences 



